This invention relates to the manufacture of fluorescent lamps and, more particularly to a method and apparatus for non-destructively measuring the internal film resistance in a fluorescent lamp.
A conductive film coating on the interior bulb wall of a fluorescent lamp provides for easier lamp starting than would be possible in a conventional fluorescent lamp. The resistance of the film must be of a specific value, however, in order for this beneficial starting effect to occur. For this reason, a means of measuring such a film without destroying the lamp is needed during production of the lamp. The conductive films referred to can be either SnO.sub.2 or In.sub.2 O.sub.3, these being the most commonly used, or any other film which is compatible with fluorescent lamp making. These films are applied to the inner wall of the bulb before the phosphor coating is applied. The bulbs are then processed into finished fluorescent lamps in a conventional manner.
In particular, these films are beneficial in starting energy saving fluorescent lamps. Such lamps use krypton gas as a constituent of the filling gas. Krypton gas is a more efficient gas than the argon gas customarily used in fluorescent lamps and it is for this reason that it is being used in energy saving lamps. An undesirable property of krypton gas, however, is the hard starting property which it imparts to a fluorescent lamp. With a conductive film of the proper resistance value, however, this objectionable property can be eliminated.
Prior to the development of the present invention, the internal film resistance of the lamp measured destructively by opening the lamp and attached probes to the bulb wall after the phosphor was removed. Such a procedure is obviously time consuming and does not permit a 100% inspection of the finished product. Further, the accuracy of the measurement as it relates to the evacuated lamp is in doubt. More specifically, with some conductive films, the presence of O.sub.2 at atmospheric pressure produces changes in the film resistance. Consequently, the film resistance measured using a destructive method will not necessarily be that of the original lamp.